Staphylococcus aureus is a common bacterium found in moist areas of the human body and skin. Approximately 29% of the US population is colonized in the nose with S. aureus, of which 1.5% is methicillin- resistant S. aureus (MRSA). Annually, 278,000 people in the US are hospitalized with MRSA infections, resulting in 19,000 deaths. Spread of MRSA is also found in community-acquired infections, with over 6 million outpatient visits every year in the US caused by MRSA. Over the years, -lactams were antibiotics of choice in treatment of S. aureus infections. However, these agents faced obsolescence with the emergence of MRSA. We have discovered the quinazolinone class of antibacterial agents, which exhibit activity against MRSA, including hard-to-treat vancomycin- and linezolid-resistant MRSA strains. The lead quinazolinone shows efficacy in animal models of infection and has oral bioavailability in mice. The quinazolinones have antibacterial activity o their own, but they also synergize with -lactam antibiotics. We have shown that the quinazolinones bind to the allosteric site in penicillin-binding protein 2a (PBP2a), an unprecedented mode of action for any antibacterial. The binding to the allosteric site triggers opening of the active site, a unique mechanism that can also be exploited to inactivate PBP2a by co-administration with -lactam antibiotics, thus resurrecting obsolete -lactam antibiotics i treatment of MRSA. Three Specific Aims are proposed for lead optimization of the quinazolinone class of antibiotics. These studies include additional mechanism of action experiments, pharmacodynamics, investigation of emergence of resistance, and evaluation of combinations of the quinazolinones with other antibiotics. These studies will chart the preclinical development of these novel antibiotics, which hold promise in treatment of infections by Gram-positive bacteria, including MRSA.